Flexible cube-corner retroreflective sheeting

ABSTRACT

A retroreflective article 10 has a body portion 14 and a multitude of cube-comer elements 12 that project from a rear side 20 of the body portion 14. The body portion 14 includes a body layer 18 that contains a light-transmissible polymeric material having an elastic modulus less than 7x108 pascals. The cube-corner elements 12 contain a light transmissible polymeric material having an elastic modulus greater than 16x108 pascals. A retroreflective article of this construction can be highly flexed while maintaining good retroreflective performance.

TECHNICAL FIELD

This invention pertains to a flexible cube-corner retroreflectivesheeting, and more particularly to a flexible cube-cornerretroreflective sheeting that uses a high elastic modulus polymer in thecube-corner elements and a low elastic modulus polymer in the bodyportion.

BACKGROUND OF THE INVENTION

Retroreflective sheetings have the ability to redirect incident lighttowards its originating source. This unique ability has led to thewide-spread use of retroreflective sheetings on a variety of articles.Very often the retroreflective sheetings are used on flat inflexiblearticles, for example, road signs and barricades; however, situationsfrequently arise which require the sheetings to be used on irregular orflexible surfaces. For example, a retroreflective sheeting may beadhered to the side of a truck trailer, which requires the sheeting topass over corrugations and protruding rivets, or the sheeting may beadhered to a flexible substrate such as a road worker's safety vest. Insituations where the underlying surface is irregular or flexible, theretroreflective sheeting desirably possesses good conformability andflexibility but not at the expense of sacrificing retroreflectiveperformance.

There are essentially two types of retroreflective sheeting: beadedsheeting and cube-corner sheeting. Beaded sheeting employs a multitudeof glass or ceramic microspheres to retroreflect incident light. Themicrospheres are separate from each other and therefore do not severelyimpose on the sheeting's ability to be flexed. Cube-corner sheeting, onthe other hand, typically employs a multitude of rigid, interconnected,cube-corner elements to retroreflect incident light. Due in part totheir interconnected nature, the shape of the cube-corner elements canbecome distorted during flexing, resulting in a loss ofretroreflectivity. The construction of cube-corner sheeting, therefore,places limits on the degree to which the sheeting can be conformed orflexed and still maintain satisfactory retroreflectivity. In the attemptto expand or remove these limits, investigators have taken manydifferent approaches to produce a cube-corner sheeting whichdemonstrates good retroreflectivity after being flexed or conformed.Examples of these different approaches have been disclosed in U.S. Pat.Nos. 3,684,348, 3,924,929, 3,992,080, 4,555,161, 4,576,850, 4,668,558,4,582,885, 5,177,304, 5,189,553 and U.K. Patent GB 2,245,219 A.

In U.S. Pat. No. 3,684,348 a retroreflective sheeting is disclosed thathas a multitude of cube-corner formations projecting from a bodyportion. The cube-corner formations and the body portion are separatelyformed from essentially transparent synthetic plastic resin and arebonded together as a composite structure. To facilitate mounting on andshaping to surfaces of various configurations, it is disclosed that thebody portion can be flexible.

U.S. Pat. No. 3,924,929 discloses a cube-corner retroreflective sheetingthat contains a multiplicity of trihedral prismatic retroreflectiveunits separated into cells by interconnected sepia. A multiplicity ofretroreflective sheeting units may be bonded to a flexible backing suchas a polyester or polyvinylchloride web. The retroreflective sheetingunits are positioned on the backing in a manner that allows the flexiblearticle to be rolled or folded in either direction along two adjacentedges.

U.S. Pat. No. 3,992,080 discloses a retroreflective sheeting thatprovides good retroreflection when stretched. The sheeting comprises afirst strip of transparent flexible synthetic resin having amultiplicity of minute cube-corner formations on one surface thereof.The cube-corner formations are bonded to a second strip of flexiblebacking material of lesser length than the first strip when in a relaxedcondition. The first strip is bonded to the second strip with thecube-corner formations disposed adjacent to the second strip. Thiscomposite retroreflective sheet material is puckered in the relaxedstate and is stretchable on a support surface with the elimination ofthe puckered condition. It is disclosed that this construction allowsthe sheeting to be stretched while avoiding distortion of thecube-corner formations.

U.S. Pat. No. 4,555,161 discloses a retroreflective sheeting thatprovides a high degree of flexibility to permit the sheeting to betailored to a wide range of applications. The retroreflective sheetingcomprises a base sheet that includes a flexible synthetic plastic sheetmaterial; a cover sheet that includes a coextensive length of flexibletransparent synthetic plastic; and a multiplicity of retroreflectivefilm pieces disposed between the base sheet and the cover sheet. Thefilm pieces are arranged as an array in a predetermined pattern, and thesheets are bonded to one another in areas between and about the filmpieces to provide a multiplicity of discrete cells in which the filmpieces are seated. A portion of each of the film pieces defines amultiplicity of minor cube-corner formations that provide theretroreflective properties to the sheeting.

U.S. Pat. Nos. 4,576,850, 4,582,885, and 4,668,558 disclose aretroreflective cube-corner sheeting that possesses good flexibility anddimensional stability. The retroreflective sheeting is made from across-linked polymer composed of (1) a plurality of hard segments ofmono- or polyvalent moieties containing one or more carbocyclic and/orheterocyclic groups and (2) a plurality of soft segments of mono- orpolyvalent moieties. The moieties of the hard segments have a majortransition temperature above 250° K., and the moieties of the softsegments have a glass transition temperature below 250° K. and have anaverage molecular weight of about 500 to 5,000.

U.S. Pat. No. 5,117,304 discloses a flexible retroreflective sheetingbased upon an optically clear, aliphatic polyurethane polymer. Theretroreflective sheeting comprises a land and an array of elements onthe land. The flexibility is imparted to the sheeting by use of analiphatic polyurethane polymer in the retroreflective elements which hasa plurality of hard chain segments of the formula --C(O)N(H)--C₆ H₁₀--N(H)C(O)--.

U.S. Pat. No. 5,189,553 discloses retroreflective cube-corner sheetingthat is suitable for bending applications. The sheet has an outersurface that is designed for tension during sheet bending and an innersurface design for compression during sheet bending. A sheet neutralbending access region is located relative to the outer and innersurfaces so that a neutral bending access exists which is substantiallyfree of stress and deformation during sheet bending. The sheet also hasa plurality of reflective cube-corner elements with surfacespurposefully located proximate to the neutral bending access. It isdisclosed that this sheeting provides enhanced retroreflectiveperformance for a given radius of curvature.

U.K. Patent Application GB 2,245,219 A discloses a flexibleretroreflective sheet material comprising relatively flexible bodymember of a transparent synthetic resin. The flexible body member hasfirst and second faces, where the first face is planar, and the secondface has closely-spaced retroreflective microprisms located thereover.The body member has a thickness from the first face to the base of themicrospheres of 5.08 to 25.4 micrometers. The microprisms have a heightof 25.4 to 254 micrometers. An adhesive coating is disposed on thesecond face over some of the microprisms, and a flexible backing memberextends over the second face and is bonded to the microprisms by theadhesive coating.

Although the above-discussed patents disclose a variety of differentconstructions for providing flexible, retroreflective, cube-cornersheeting, some of the disclosed constructions are relatively complicatedin construction. Others, while being no more complicated than a typicalretroreflective sheeting, use relatively expensive polymeric materialsor provide limited retroreflectance when highly flexed or conformed, orsimply fail to fully teach how good retroreflectivity is achieved afterthe sheeting has been flexed or conformed.

SUMMARY OF THE INVENTION

The present invention provides a new approach to producing a flexiblecube-corner retroreflective sheeting. The sheeting of this invention isrelatively simple in construction when compared to some of the prior artflexible cube-corner retroreflective sheetings, and it can beextraordinarily flexed and conformed while maintaining a high degree ofretroreflectance. The sheeting also can be made from relativelyinexpensive polymers. In brief summary, the cube-corner retroreflectivesheeting of this invention comprises: a body portion that includes abody layer which contains a light transmissible polymeric materialhaving an elastic modulus less than 7×10⁸ pascals; and a plurality ofcube-corner elements projecting from a first side of the body portion,the cube-corner elements comprising a light transmissible polymericmaterial having an elastic modulus greater than 16×10⁸ pascals.

The present invention differs from known cube-corner retroreflectivesheetings in that the cube-corner elements are made from a high elasticmodulus polymeric material, and the body portion is made from a lowelastic modulus polymeric material. The term "elastic modulus" means theelastic modulus determined according to ASTM D 882-75b using StaticWeighing Method A with a five inch initial grip separation, a one inchsample width, and an inch per minute rate of grip separation.

As indicated above, retroreflective sheetings are known which employdifferent polymers in the cubes and body portion of the sheeting, see,for example, U.S. Pat. Nos. 5,117,304 and 3,684,348; however, it isbelieved that no document discloses the combination of high elasticmodulus cubes and a low elastic modulus body layer. This combination isvery advantageous because the sheeting can be highly flexed withoutsuffering from a substantial loss of retroreflectivity. The flexibilitydemonstrated by sheetings of the invention is great enough to allow thesheeting to be adhered to a highly conformed surface (for example, overa protruding rivet) by use of a conventional pressure sensitiveadhesive. The cube-comer elements demonstrate extraordinary dimensionalstability during flexing and thereby provide good retroreflectiveperformance when conformed. The dimensional stability and goodretroreflective performance can be maintained at high temperatures.

The above and other advantages of the invention are more fully shown anddescribed in the drawings and detailed description of this invention,where like reference numerals are used to represent similar parts. It isto be understood, however, that the description and drawings are for thepurposes of illustration only and should not be read in a manner thatwould unduly limit the scope of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a cube-corner retroreflectivesheeting 10 in accordance with the present invention.

FIG. 2 is a view of the backside of the cube-corner retroreflectivesheeting 10 of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In describing preferred embodiments of the invention, specificterminology will be used for the sake of clarity. The invention,however, is not intended to be limited to the specific terms soselected, and it is to be understood that each term so selected includesall technical equivalents that operate similarly.

In the practice of the present invention, a cube-corner retroreflectivesheeting is provided which maintains good dimensional stability and highdegrees of retroreflectance under highly flexed conditions. In FIG. 1,there is shown an example of a cube-corner retroreflective sheeting 10of the present invention. Cube-corner retroreflective sheeting 10comprises a multitude of cube-corner elements 12 and a body portion 14.The body portion 14 can include a land layer 16 and a body layer 18. Thebody layer typically functions to protect the sheeting fromenvironmental elements and/or to provide significant mechanicalintegrity to the sheeting. In a preferred embodiment, the body layer 18is the outermost layer on the front side of the sheeting 10. The landlayer 16 is distinguished from the body layer 18 by being a layerdisposed immediately adjacent to the base of the cube-corner elements,and the term "land layer" is used herein to mean such a layer.

The cube-corner elements 12 project from a first or rear side 20 of bodyportion 14. The cube-comer elements 12 comprise a light transmissiblepolymeric material having an elastic modulus greater than 16×10⁸pascals, and the body layer 18 comprises a light transmissible polymericmaterial having an elastic modulus less than 7×10⁸ pascals. Light entersthe cube-corner sheeting 10 through the front surface 21. The light thenpasses through the body portion 14 and strikes the planar faces 22 ofthe cube-corner elements 12 and returns in the direction from which itcame as shown by arrow 23.

In a preferred construction, the cube-corner elements 12 and land layer16 are made from similar or the same kind of polymers, and the landlayer 16 is kept to a minimal thickness. The land layer 16, typically,has a thickness in the range of about 0 to 150 micrometers, andpreferably in the range of approximately about 1 to 100 micrometers.Body layer 18 typically has a thickness of approximately 20 to 1,000micrometers, and preferably in the range of about 50 to 250 micrometers.Although it is preferred to keep the land layer to a minimum thickness,it is desired that the sheeting 10 possess some land layer 16 so that afiat interface can be provided between the land layer 16 and the bodylayer 18. The cube-corner elements 12 typically have a height in therange of about 20 to 500 micrometers, and more typically in the range ofabout 60 to 180 micrometers. Although the embodiment of the inventionshown in FIG. 1 has a single body layer 18, it is within the scope ofthe present invention to provide more than one body layer 18 in the bodyportion 14.

FIG. 2 illustrates the back side of the cube-corner elements 12. Asshown, the cube-corner elements 12 are disposed as matched pairs in anarray on one side of the sheeting. Each cube-corner element 12 has theshape of a trihedral prism with three exposed planar faces 22. Theplanar faces 22 may be substantially perpendicular to one another (as inthe corner of a room) with the apex 24 of the prism vertically alignedwith the center of the base. The angle between the faces 22 typically isthe same for each cube-corner element in the array and will be about90°. The angle, however, can deviate from 90° as is well-known; see, forexample, U.S. Pat. No. 4,775,219 to Appledorn et al. Although the apex24 of each cube-corner element 12 may be vertically aligned with thecenter of the base of the cube-corner element, see, for example, U.S.Pat. No. 3,684,348. The apex also may be canted to the center of thebase as disclosed in U.S. Pat. No. 4,588,258. Thus, the presentinvention is not limited to any particular cube-corner geometry;however, of the many known cube-corner configurations, see, for example,U.S. Pat. Nos. 4,938,563, 4,775,219, 4,243,618, 4,202,600, and3,712,706, the cube-corner sheeting described in U.S. Pat. No. 4,588,258may be preferred because it provides wide angle retroreflection amongmultiple viewing planes.

A specular reflective coating such as a metallic coating (not shown) canbe placed on the backside of the cube-corner elements 12 to promoteretroreflection. The metallic coating can be applied by known techniquessuch as vapor depositing or chemically depositing a metal such asaluminum, silver, or nickel. A primer layer may be applied to thebackside of the cube-corner elements to promote the adherence of themetallic coating. In addition to or in lieu of a metallic coating, aseal film can be applied to the backside of the cube-corner elements;see, for example, U.S. Pat. Nos. 4,025,159 and 5,117,304. The sealingfilm maintains an air interface at the backside of the cubes to enhanceretroreflectivity. A backing or an adhesive layer also can be disposedbehind the cube-corner elements to enable the cube-cornerretroreflective sheeting 10 to be secured to a substrate.

The polymeric materials that compose the retroreflective sheeting of theinvention are light transmissible. This means that the polymer is ableto transmit at least 70 percent of the intensity of the light incidentupon it at a given wavelength. More preferably, the polymers that areused in the retroreflective sheeting of the invention have a lighttransmissibility of greater than 80 percent, and more preferably greaterthan 90 percent.

The polymeric materials that are employed in the cube-corner elementstend to be hard and rigid. The polymeric materials may be thermoplasticor crosslinkable resins. The elastic modulus of these polymerspreferably is greater than 18×10⁸ pascals, and more preferably isgreater than 20×10⁸ pascals.

When thermoplastic polymers are used in the cubes, the glass transitiontemperature generally is greater than 80° C., and the softeningtemperature is typically greater than 150° C. Generally, thethermoplastic polymers used in the cube-corner layer are amorphous orsemi-crystalline, and the linear mold shrinkage of the polymer generallyis less than one percent.

Examples of thermoplastic polymers that may be used in the cube-cornerelements include acrylic polymers such as poly(methyl methacrylate);polycarbonates; cellulosics such as cellulose acetate, cellulose(acetate-co-butyrate), cellulose nitrate; epoxies; polyesters such aspoly(butylene terephthalate), poly(ethylene terephthalate);fluoropolymers such as poly(chlorofluoroethylene), poly(vinylidenefluororide); polyamides such as poly(caprolactam), poly(amino caproicacid), poly(hexamethylene diamine-co-adipic acid), poly(amide-co-imide),and poly(ester-co-imide); polyetherketones; poly(etherimide);polyolefins such as poly(methylpentene); poly(phenylene ether);poly(phenylene sulfide); poly(styrene) and poly(styrene) copolymers suchas poly(styrene-co-acrylonitrile),poly(styrene-co-acrylonitrile-co-butadiene); polysulfone; siliconemodified polymers (i.e., polymers that contain a small weight percent(less than 10 weight percent) of silicone) such as silicone polyamideand silicone polycarbonate; fluorine modified polymers such asperfluoropoly(ethyleneterephthalate); and mixtures of the above polymerssuch as a poly(ester) and poly(carbonate) blend, and a fluoropolymer andacrylic polymer blend.

Additional materials suitable for forming the cube-corner elements arereactive resin systems capable of being crosslinked by a free radicalpolymerization mechanism by exposure to actinic radiation, for example,electron beam, ultraviolet light, or visible light. Additionally, thesematerials may be polymerized by thermal means with the addition of athermal initiator such as benzoyl peroxide. Radiation-initiatedcationically polymerizable resins also may be used.

Reactive resins suitable for forming the cube-corner elements may beblends of photoiniator and at least one compound bearing an acrylategroup. Preferably the resin blend contains a difunctional orpolyfunctional compound to ensure formation of a crosslinked polymericnetwork upon irradiation.

Examples of resins that are capable of being polymerized by a freeradical mechanism include acrylic-based resins derived from epoxies,polyesters, polyethers and urethanes, ethylenically unsaturatedcompounds, aminoplast derivatives having at least one pendant acrylategroup, isocyanate derivatives having at least one pendant acrylategroup, epoxy resins other than acrylated epoxies, and mixtures andcombinations thereof. The term acrylate is used here to encompass bothacrylates and methacrylates. U.S. Pat. No. 4,576,850 to Martens(disclosure incorporated here by reference) discloses examples ofcrosslinked resins that may be used in the cube-corner elements of thepresent invention.

Ethylenically unsaturated resins include both monomeric and polymericcompounds that contain atoms of carbon, hydrogen and oxygen, andoptionally nitrogen, sulfur and the halogens. Oxygen or nitrogen atomsor both are generally present in ether, ester, urethane, amide and ureagroups. Ethylenically unsaturated compounds preferably have a molecularweight of less than about 4,000 and preferably are esters made from thereaction of compounds containing aliphatic monohydroxy groups oraliphatic polyhydroxy groups and unsaturated carboxylic acids, such asacrylic acid, methacrylic acid, itaconic acid, crotonic acid,isocrotonic acid, maleic acid, and the like.

Some examples of compounds having an acrylic or methacrylic group arelisted below. The listed compounds are illustrative and not limiting.

(1) Monofunctional compounds: ethylacrylate, n-butylacrylate,isobutylacrylate, 2-ethylhexylacrylate, n-hexylacrylate,n-octylacrylate, isobornyl acrylate, tetrahydrofurfuryl acrylate,2-phenoxyethyl acrylate, N,N-dimethylacrylamide;

(2) Difunctional compounds: 1,4-butanediol diacrylate, 1,6-hexanedioldiacrylate, neopentylglycol diacrylate, ethylene glycol diacrylate,triethyleneglycol diacrylate, and tetraethylene glycol diacrylate;

(3) Polyfunctional compounds: trimethylolpropane triacrylate,glyceroltriacrylate, pentaerythritol triacrylate, pentaerythritoltetraacrylate, and tris(2-acryloyloxyethyl)isocyanurate.

Some representative examples of other ethylenically unsaturatedcompounds and resins include styrene, divinylbenzene, vinyl toluene,N-vinyl pyrrolidone, N-vinyl caprolactam, monoallyl, polyallyl, andpolymethallyl esters such as diallyl phthalate and diallyl adipate, andamides of carboxylic acids such as and N,N-diallyladipamide.

Examples of photopolymerization initiators which can be blended with theacrylic compounds include the following illustrative initiators: benzil,methyl o-benzoate, benzoin, benzoin ethyl ether, benzoin isopropylether, benzoin isobutyl ether, etc., benzophenone/tertiary amine,acetophenones such as 2,2-diethoxyacetophenone, benzil methyl ketal,1-hydroxycyclohexyl phenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one,2-benzyl-2-N,N-dimethylamino-1-(4-morpholinophenyl)-1-butanone,2,4,6-trimethylbenzoyldiphenylphosphine oxide,2-methyl-1-4-(methylthio)phenyl-2-morpholino-1-propanone, et cetera.These compounds may be used individually or in combination.

Cationically polymerizable materials include but are not limited tomaterials containing epoxy and vinyl ethers functional groups. Thesesystems are photoinitiated by onium salt initiators such astriarylsulfonium, and diaryliodonium salts.

Preferred polymers for the cube-corner elements include poly(carbonate),poly(methylmethacrylate), poly(ethyleneterephthalate), and crosslinkedacrylates such as multi-functional acrylates or epoxies and acrylatedurethanes blended with mono- and multi-functional monomers. Thesepolymers are preferred for one or more of the following reasons: thermalstability, environmental stability, clarity, excellent release from thetooling or mold, and capable of receiving a reflective coating.

The polymeric materials employed in the land layer, as indicated above,may be the same as the polymers that are employed in the cube-cornerelements, provided that the land layer is kept to a minimal thickness.The land layer preferably is substantially flat so that a betterinterface is achieved between the cubes and the body layer. Cavitiesand/or interfacial roughness preferably are avoided between the cubesand land layer so that optimum brightness can be displayed by theretroreflective sheeting when light is retroreflected therefrom. A goodinterface prevents spreading of retroreflective light from refraction.In most instances, the land layer is integral with the cube-cornerelements. By "integral" is meant the land and cubes are formed from asingle polymeric material--not two different polymeric layerssubsequently united together. The polymers that are employed in thecube-corner elements and land layer can have refractive indices whichare different from the body layer. Although the land layer desirably ismade of a polymer similar to that of the cubes, the land also may bemade from a softer polymer such as those used in the body layer.

The body layer comprises a low elastic modulus polymer for easy bending,curling, flexing, conforming, or stretching. The elastic moduluspreferably is less than 5×10⁸ pascals, and more preferably is less than33×10⁸ pascals. Generally, the polymers of the body layer have a glasstransition temperature that is less than 50° C. The polymer preferablyis such that the polymeric material retains its physical integrity atthe temperatures at which it is applied to the cubes. The polymerdesirably has a vicate softening temperature that is greater than 50° C.The linear mold shrinkage of the polymer desirably is less than 1percent. Preferred polymeric materials used in the body layer areresistant to degradation by UV light radiation so that theretroreflective sheeting can be used for long-term outdoor applications.Examples of polymers that may be employed in the body layer include:

fluorinated polymers such as: poly(chlorotrifluoroethylene), for exampleKel-F800™ available from 3M, St. Paul, Minn.;poly(tetrafluoroethylene-co-hexafluoropropylene), for example Exac FEP™available from Norton Performance, Brampton, Mass.;poly(tetrafluoroethylene-co-perfluoro(alkyl)vinylether), for example,Exac PEA™ also available from Norton Performance; and poly(vinylidenefluoride-co-hexafluoropropylene), for example, Kynar Flex-2800™available from Pennwalt Corporation, Philadelphia, Pa.;

ionomeric ethylene copolymers such as: poly(ethylene-co-methacrylicacid) with sodium or zinc ions such as Surlyn-8920™ and Surlyn-9910™available from E.I. dupont Nemours, Wilmington, Del.;

low density polyethylenes such as: low density polyethylene; linear lowdensity polyethylene; and very low density polyethylene;

plasticized vinyl halide polymers such as plasticizedpoly(vinylchloride);

polyethylene copolymers including: acid functional polymers such aspoly(ethylene-co-acrylic acid) and poly(ethylene-co-methacrylic acid)poly(ethylene-co-maleic acid), and poly(ethylene-co-fumaric acid);acrylic functional polymers such as poly(ethylene-co-alkylacrylates)where the alkyl group is methyl, ethyl, propyl, butyl, et cetera, or CH₃(CH₂)n-- where n is 0-12, and poly(ethylene-co-vinylacetate); and

aliphatic and aromatic polyurethanes derived from the following monomers(1)-(3): (1) diisocyanates such asdicyclohexylmethane-4,4'-diisocyanate, isophorone diisocyanate,1,6-hexamethylene diisocyanate, cyclohexyl diisocyanate, diphenylmethanediisocyanate, and combinations of these diisocyanates, (2) polydiolssuch as polypentyleneadipate glycol, polytetramethylene ether glycol,polyethylene glycol, polycaprolactone diol, poly-1,2-butylene oxideglycol, and combinations of these polydiols, and (3) chain extenderssuch as butanediol or hexanediol. Commercially available urethanepolymers include: PN-04, or 3429 from Morton International Inc.,Seabrook, N.H., or X-4107 from B.F. Goodrich Company, Cleveland, Ohio.

Combinations of the above polymers also may be employed in the bodylayer of the body portion. Preferred polymers for the body layerinclude: the ethylene copolymers that contain units that containcarboxyl groups or esters of carboxylic acids such aspoly(ethylene-co-acrylic acid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinylacetate); the ionomeric ethylene copolymers;plasticized poly(vinylchloride); and the aliphatic urethanes. Thesepolymers are preferred for one or more of the following reasons:suitable mechanical properties, good adhesion to the land layer,clarity, and environmental stability.

In an embodiment that contains polycarbonate cube-corner elements and/ora polycarbonate land layer and a body layer that contains a polyethylenecopolymer such as poly(ethylene-co-(meth)acrylic acid),poly(ethylene-co-vinylacetate) or poly(ethylene-co-acrylate), theinterfacial adhesion between the body layer and the land layer orcube-corner elements can be improved by placing a thin tie-layer (notshown) therebetween. The tie-layer can be applied on the body layerbefore laminating the body layer to the land layer or to the cube-cornerelements. The tie-layer can be applied as a thin coating using, forexample: an aliphatic polyurethane in organic solution, for examplePermuthane™ U26-248 solution, available from Permuthane Company,Peabody, Mass.; Q-thane™ QC-4820 available from K. J. Quinn and Co.,Inc., Seabrook, N.H.; an aliphatic polyurethane waterborne dispersion,for example NeoRez™ R-940, R-9409, R-960, R-962, R-967, and R-972,available from ICI Resins US, Wilmington, Mass.; an acrylic polymerwater borne dispersion, for example, NeoCryl™ A-601, A-612, A-614,A-621, and A-6092, available from ICI Resins US, Wilmington, Mass.; oran alkyl acrylate and aliphatic urethane copolymer water bornedispersion, for example NeoPac™ R-9000, available from ICI Resins US,Wilmington, Mass. In addition, an electrical discharge method, such as acorona or plasma treatment, can be used to further improve the adhesionof tie-layer to the body layer or the tie-layer to the land layer or tothe cube-corner elements.

Colorants, UV absorbers, light stabilizers, free radical scavengers orantioxidants, processing aids such as antiblocking agents, releasingagents, lubricants, and other additives may be added to the body portionor cube-corner elements. The particular colorant selected, of course,depends on the desired color of the sheeting. Colorants typically areadded at about 0.01 to 0.5 weight percent. UV absorbers typically areadded at about 0.5 to 2.0 weight percent. Examples of UV absorbersinclude derivatives of benzotriazole such as Tinuvin™ 327, 328, 900,1130, Tinuvin-P™, available from Ciba-Geigy Corporation, Ardsley, N.Y.;chemical derivatives of benzophenone such as Uvinul™-M40, 408,D-50,available from BASF Corporation, Clifton, N.J.; Syntase™ 230, 800, 1200available from Neville-Synthese Organics, Inc., Pittsburgh, Pa.; orchemical derivatives of diphenylacrylate such as Uvinul™-N35, 539, alsoavailable from BASF Corporation of Clifton, N.J. Light stabilizers thatmay be used include hindered amines, which are typically used at about0.5 to 2.0 weight percent. Examples of hindered amine light stabilizersinclude Tinuvin™-144,292, 622, 770, and Chimassorb™-944 all availablefrom the Ciba-Geigy Corp., Ardsley, N.Y. Free radical scavengers orantioxidants may be used, typically, at about 0.01 to 0.5 weightpercent. Suitable antioxidants include hindered phenolic resins such asIrganox™-1010, 1076, 1035, or MD-1024, or Irgafos™-168, available fromthe Ciba-Geigy Corp., Ardsley, N.Y. Small amount of other processingaids, typically no more than one weight percent of the polymer resins,may be added to improve the resin's processibility. Useful processingaids include fatty acid esters, or fatty acid amides available fromGlyco Inc., Norwalk, Conn., metallic stearates available from HenkelCorp., Hoboken, N.J., or Wax E™ available from Hoechst CelaneseCorporation, Somerville, N.J.

Cube-corner retroreflective sheetings of the invention can be made by:(a) forming a plurality of cube-corner elements from a lighttransmissible material having an elastic modulus greater than 16×10⁸pascals; and (b) securing a body layer to the plurality of cube-cornerelements, wherein the body layer includes a light transmissible materialhaving an elastic modulus less than 7×10⁸ pascals. Steps (a) and (b) canbe carried out according to a variety of known (or later discovered)methods for making cube-corner sheeting, see, for example, U.S. Pat.Nos. 3,689,346, 3,811,983, 4,332,847, and 4,601,861, with the exceptionof using a high elastic modulus polymer to form the cube-comer elementsand a low elastic modulus polymer to form the body layer. The body layermay be secured directly to the base of the cube-corner elements, or itmay be secured to the cube-corner elements by a land layer. As indicatedabove, the land layer preferably is kept to a minimal thickness andpreferably is made from a high elastic modulus material.

Features and advantages of this invention are further illustrated in thefollowing examples. It is to be expressly understood, however, thatwhile the examples serve this purpose, the particular ingredients andamounts used as well as other conditions and details are not to beconstrued in a manner that would unduly limit the scope of thisinvention.

EXAMPLES

Example 1 (Comparative)

Molten polycarbonate resin (MakroIon™ 2407, supplied by MobayCorporation, Pittsburgh, Pa.) was cast onto a heated microstructurednickel tooling containing microcube prism recesses having a depth ofapproximately 86 micrometers (0.0034 inch). The microcube recesses wereformed as matched pairs of cube-corner elements with the optical axiscanted or tilted 4.31 degrees away from the primary groove, as generallyillustrated in U.S. Pat. No. 5,138,488 to Szczech. The nickel toolingthickness was 508 micrometers (0.020 inch), and the tooling was heatedto 215.6° C. (420° F.). Molten polycarbonate at a temperature 287.8° C.(550° F.) was cast onto the tooling at a pressure of approximately1.03×10⁷ to 1.38×10⁷ pascals (1500 to 2000 psi) for 0.7 seconds in orderto replicate the microcube recesses. Coincident with filling the cuberecesses, additional polycarbonate was deposited in a continuous landlayer above the tooling with a thickness of approximately 51 micrometers(0.002 inch). A previously extruded 43 micrometer (0.0017 inch) thickimpact modified, continuous, poly(methylmethacrylate) body layer film(Plexiglass 60% VO-45 and 40% DR-1000, material supplied by Rohm andHaas Company, Philadelphia, Pa.) was then laminated onto the top surfaceof the continuous polycarbonate land layer when the surface temperaturewas approximately 190.6° C. (375° F.). The combined tooling withlaminated poly(carbonate) and poly(methylmethacrylate) body layer wasthen cooled with room temperature air for 18 seconds to a temperature of71.1-87.8 C. (160-190 F.), allowing the laminate materials to solidify.The laminate sample was then removed from the microstructured tool.

Example 2

A second sample was produced by the procedure outlined in Example 1,except the body layer was a previously extruded 79 micrometer (0.0031inch) thick, continuous, poly(ethylene-co-acrylic acid) film (Primacor™3440, material supplied by The Dow Chemical Company, Midland, Mich.). Analiphatic polyester urethane primer was applied to promote adhesion ofthis film to the poly(carbonate) surface. The primer (Q-Thane™ QC-4820,supplied by K. J. Quinn and Co., Inc., Seabrook, N.H., was solventcoated to form a layer having a final dried thickness of approximately2.5 micrometers (0.0001 inch).

Example 3

A third sample was produced by the procedure outlined in Example 1,except the body layer was a previously extruded 71 micrometer (0.0028inch) thick aliphatic polyester urethane body layer (Morthane™ PNO3,supplied by Morton International, Seabrook, N.H.).

Table 1 summarizes the elastic modulus of the body layer material forthe laminate samples evaluated in Examples 1-3. Elastic modulus wasdetermined according to ASTM D882-75b using a Sintech 1 static weighingconstant rate-of-grip separation tester, produced by MTS Systems Corp,Eden Prairie, Minn. Elastic modulus for the poly(carbonate) used in thecubes and land layer for all samples was 20.0×10⁸ pascals (29.1×10⁴psi).

                  TABLE 1                                                         ______________________________________                                        Example                                                                       Number  Body Layer Material                                                                              Elastic Modulus                                    ______________________________________                                        1       polymethylmethacrylate                                                                           24.4 × 10.sup.8 pascal                                                  (35.4 × 10.sup.4 psi)                        2       poly(ethylene-co-acrylic acid)                                                                   1.24 × 10.sup.8 pascal                                                  (1.80 × 10.sup.4 psi)                        3       aliphatic polyester urethane                                                                     0.34 × 10.sup.8 pascal                                                  (0.50 × 10.sup.4 psi)                        ______________________________________                                    

The samples of Examples 1-3 were tested for apparent bending modulususing a Taber™ V-5 stiffness tester, manufactured by Taber InstrumentCorp, North Tonawanda, N.Y. Calculations were performed in accordancewith ASTM D747-84a. The results are summarized in Table 2.

The samples of Examples 1-3 also were tested for optical path difference(OPD) resulting from bending to a radius of curvature of 9.52millimeters (0.375 inch). OPD measurements were obtained using a phaseshifting Twyman-Green interferometer (manufactured by WYKO Corporation,Tucson, Ariz.) operating at a wavelength of 633 nanometers. OPD providesa means of quantifying the optical distortion of a wave front as it isretroreflected by a cube corner element. Increased optical distortiontranslates directly into increased spreading of the retroreflected lightpattern or divergence profile using fourier optics (for example, asdiscussed in "The New Physical Optics Notebook: Tutorials in FourierOptics", Reynolds, DeVelis, Parrent, and Thompson, SPIE Press 1989).Samples or constructions which minimize the OPD wave front distortionfor a given deformation are preferred.

Three cube-corner elements were selected on samples representing each ofthe three examples. The OPD for each of the three cubes, with units ofnumber of wavelengths, was measured both before and after bending aboutan axis parallel to the primary groove. The OPD results were thendifferenced to provide an OPD associated only with the bending for eachof the three cubes, and the difference results were averaged to producea final average OPD associated with bending of each of the samples. OPDpeak to valley (PV) and root mean squared (RMS) were used to quantifythe wave front distortion. P-V and RMS OPD results are summarized inTable 2.

                  TABLE 2                                                         ______________________________________                                               Apparent Bending                                                       Example                                                                              Modulus        P-V OPD.sup.a                                                                           RMS OPD.sup.a                                 ______________________________________                                        1      8.62 × 10.sup.8 pascal                                                                 1.12      0.27                                                 (1.25 × 10.sup.5 psi)                                            2      1.73 × 10.sup.8 pascal                                                                 0.72      0.14                                                 (0.25 × 10.sup.5 psi)                                            3      1.24 × 10.sup.8 pascal                                                                 0.67      0.12                                                 (0.18 × 10.sup.5 psi)                                            ______________________________________                                         .sup.a OPD changes associated with bending to a radius of curvature of        9.52 millimeters (0.375 inch)                                            

The data in Table 2 demonstrates that the use of a relatively lowmodulus body layer film in combination with high modulus cube materialscan produce a significantly more flexible laminate construction.Surprisingly, the resulting flexible laminate also exhibits a muchsmaller reduction in optical performance for a given deformation whencompared to a conventional laminate with a high modulus body layer.

This invention may take on various modifications and alterations withoutdeparting from the spirit and scope thereof. Accordingly, it is to beunderstood that this invention is not to be limited to theabove-described but is to be controlled by the limitations set forth inthe following claims and any equivalents thereof.

What is claimed is:
 1. A retroreflective article that comprises:a bodyportion that includes a body layer which contains a light transmissiblepolymeric material having an elastic modulus less than 7×10⁸ pascals;and a plurality of cube-corner elements projecting from a first side ofthe body portion, the cube-corner elements comprising a lighttransmissible polymeric material having an elastic modulus greater than16×10⁸ pascals.
 2. The retroreflective article of claim 1, wherein thebody portion includes a land layer that has a thickness in the range of0 to 150 micrometers and is comprised of a light transmissible polymericmaterial having an elastic modulus greater than 16×10⁸ pascals.
 3. Theretroreflective article of claim 2, wherein the land layer has athickness in the range of 1 to 100 micrometers.
 4. The retroreflectivearticle of claim 3, wherein the cube-corner elements and land layercomprise a polymer(s) that has an elastic modulus greater than 18×10⁸pascals.
 5. The retroreflective article of claim 1, wherein the bodylayer has a thickness of approximately 20 to 1,000 micrometers.
 6. Theretroreflective article of claim 5, wherein the body layer has athickness in the range of 50 to 250 micrometers.
 7. The retroreflectivearticle of claim 6, wherein the cube-corner elements have a height inthe range of about 60 to 180 micrometers.
 8. The retroreflective articleof claim 1, wherein the cube-corner elements comprise a polymericmaterial having an elastic modulus of greater than 18×10⁸ pascals. 9.The retroreflective article of claim 8, wherein the cube-corner articlescomprise a light transmissible polymeric material having a elasticmodulus greater than 20×10⁸ pascals.
 10. The retroreflective article ofclaim 1, wherein the cube-corner elements contain poly(carbonate),poly(methylmethacrylate), poly(ethyleneterephthalate), or a crosslinkedacrylate.
 11. The retroreflective cube-corner article of claim 10,wherein the body portion includes a land layer that comprises the samepolymeric material as the cube-corner elements.
 12. The retroreflectivearticle of claim 1, wherein the body layer contains a lighttransmissible polymeric material having an elastic modulus less than5×10⁸ pascals.
 13. The retroreflective article of claim 12, wherein thebody layer contains a light transmissible polymeric material that has anelastic modulus less than 3×10⁸ pascals.
 14. The retroreflective articleof claim 1, wherein the body layer contains: an ethylene copolymer thatcontains units that contain carboxyl groups or esters of carboxylicacids, ionomeric ethylene copolymers; plasticized poly(vinylchloride);an aliphatic urethane or combinations thereof.
 15. The retroreflectivearticle of claim 14, wherein the body layer contains aliphatic urethanesthat contain: units of polyester glycol, polyether glycol, polycarbonateglycol, poly-1,2-butylene oxide glycol, or combinations thereof; andunits of dicyclohexylmethane-4,4'-diisocyanate, isophorone diisocyanate,1,6-hexamethylene diisocyanate, cyclohexyl diisocyanate, or combinationsthereof.
 16. The retroreflective article of claim 14, wherein theethylene copolymers that contain units that contain carboxyl groups oresters of carboxylic acids are selected from the group consisting ofpoly(ethylene-co-acrylic acid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinylacetate), and combinations thereof.
 17. Theretroreflective article of claim 1, wherein the body layer contains alight transmissible polymeric material having a thickness of 50 to 250micrometers and an elastic modulus of less than 5×10⁸ pascals, and theplurality of cube-corner elements contain a light-transmissiblepolymeric material having an elastic modulus greater than 18×10⁸pascals.
 18. The retroreflective article of claim 17, wherein the bodylayer contains a polymer selected from the group consisting ofpoly(ethylene-co-acrylic acid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinyl acetate), an ionomeric ethylene copolymer, and analiphatic urethane; and wherein the cube-corner elements contain apolymer selected from the group consisting of poly(carbonate),poly(methylmethacrylate), poly(ethyleneterephthalate), and crosslinkedacrylates.
 19. The retroreflective article of claim 18, wherein the bodylayer comprises poly(ethylene-co-acrylic acid) orpoly(ethylene-co-methacrylic acid), and the cube-corner elements containpolycarbonate.
 20. The retroreflective article of claim 19, wherein thebody portion includes a land layer that contains polycarbonate, andfurther comprises a tie layer disposed between the body layer and landlayer, which tie layer contains an aliphatic polyurethane.
 21. Theretroreflective article of claim 1, wherein the body layer providessignificant mechanical integrity to the retroreflective article.
 22. Theretroreflective article of claim 1, wherein the body layer containsultra-violet light absorbers, light stabilizers, free radicalscavengers, or combinations thereof.
 23. A method of making aretroreflective article, which method comprises:(a) forming a pluralityof cube-corner elements from a light transmissible material having anelastic modulus greater than 16×10⁸ pascals; and (b) securing a bodylayer to the plurality of cube-corner elements, wherein the body layercontains a light transmissible material having an elastic modulus lessthan 7×10⁸ pascals.
 24. The method of claim 23, wherein body layercontains a polymer selected from the group consisting ofpoly(ethylene-co-acrylic acid), poly(ethylene-co-methacrylic acid),poly(ethylene-co-vinyl acetate), an ionomeric ethylene copolymer, and analiphatic urethane; and wherein the cube-corner elements contain apolymer selected from the group consisting of poly(carbonate),poly(methylmethacrylate), poly(ethyleneterephthalate), and crosslinkedacrylates.
 25. The method of claim 24, wherein the body layer comprisespoly(ethylene-co-acrylic acid) or poly(ethylene-co-methacrylic acid),and the cube-corner elements contain polycarbonate.
 26. The method ofclaim 25, wherein the body portion includes a land layer that containspolycarbonate, and further comprises a tie layer disposed between thebody layer and land layer, which tie layer contains an aliphaticpolyurethane.
 27. The method of claim 23, wherein the body layercontains a light transmissible polymeric material having a thickness of50 to 250 micrometers and an elastic modulus of less than 5×10⁸ pascals,and the plurality of cube-corner elements contain a light-transmissiblepolymeric material having an elastic modulus greater than 18×10⁸pascals.
 28. The method of claim 23, further comprising:forming a landlayer from a light transmissible material having an elastic modulusgreater than 16×10⁸ pascals; and securing the land layer to thecube-corner elements such that the land layer is disposed between thecube-corner elements and the body layer.